ValueStringBuilder.Append("") should be a noop

[WhiteBlackGoose]

This is useful for example for string.Join, where most people would do string.Join("", blabla) to join strings without any delimiter, but it has a small performance penalty compared to string.Join(null, blabla)

[stephentoub]

This makes all non-empty inserts more expensive in exchange for making empty appends a tad cheaper. Can you share data that demonstrates that's a worthwhile tradeoff? My expectation is it would not be, that it's much more common to append non-empty strings.

[neon-sunset]

A cheaper way to handle this could be by detecting an empty separator upfront and deferring to .Concat. I don't see this being handled on the path to JoinCore so presume it's not done at the moment: https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs#L879-L908

[MihaZupan]

Join("", ...) seems suprisingly common for some reason:

• https://github.com/search?q=%22string.Join%28%5C%22%5C%22%22+language%3AC%23&type=code&ref=advsearch
• https://github.com/search?q=%22string.Join%28string.Empty%22+language%3AC%23&type=code&ref=advsearch&l=C%23

That seems like it could be an analyzer instead

[EgorBo]

IsKnownConstant might help as well

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[jeffhandley]

Closing the PR, replaced by an analyzer proposal: Analyzer Proposal: Suggest String.Concat instead of String.Join with an empty separator (#122889)